Fast rise time quenching spark gap

ABSTRACT

Electrically floating wire mesh or perforate plate electrodes are interposed between the two electrodes of a conventional spark gap, with interelectrode spacings from 0.010 to 0.005 inches. Rise times of the order of 2 nanoseconds with repetition rates up to 200 kHz are produced.

United States Patent 1191 Austin [451 Nov. 5, 1974 FAST RISE TIMEQUENCHING SPARK GAP OTHER PUBLICATIONS [75] Inventor: William E. Austin,Wayne, Pa.

Morecroft, J. 1-1., Principles of Radio Communica- [73] Ass1gnee.General Electric Company, New on, John Wiley & Sons Inc 1927 York, NY.

22 F1 d: 1 16 May 17 1973 Primary Examiner-James W. Lawrence PP 361,196Assistant Examiner-Wm. H. Punter Attorney, Agent, or Firm-Henry W.Kaufmann; Allen 52 11.8. C1. 313/195, 313/299 Raymnd Q1115 [51] Int. Cl.H0lj 1/46 [58] Field of Search 313/195, 297, 299, 300,

313/306, 307, D16. 5, 323, 324; 331/127; [57] ABSTRACT Electricallyfloating wire mesh or perforate plate elec- [56] References Cited trodesare interposed between the two electrodes of a UNITED STATES PATENTSconventional spark gap, with interelectrode spacings from 0.010 to 0.005inches. Rise times of the order of 12112133 1311333313L151:1:111::33111111113111133123: iii/i5? 2 with ttptttttttt rates upto 200 kHz 3,349,283 10/1967 Krefft 313/195 P FOREIGN PATENTS O11APPLICATIONS 2 Claims 2 Drawing Figures 578,664 7/1946 Great Bntain315/36 V a P07 2 57 /A2 N/f J- JOURCE 1 FAST RISE TIME QUENCIIING SPARKGAP This invention was made in the performance of N000l4-73-C-0042 withthe Office of Naval Research of the Department of the Navy.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionpertains to quenched spark gaps.

2. Description of the Prior Art The quenched spark gap was first appliedto overcome certain defects of the straight gap in radio telegraphy. Inthat application it was used to shock excite an oscillating circuitcomprising a charged high-voltage capacitor and an inductor which waseither the primary of an oscillation transformer or a part of asinglewinding helix. In either case, the antenna was inductively coupledto that inductor either by the separate secondary of the oscillationtransformer or by tapped connections to a de facto secondary formed bypart of the helix; and the oscillatory energy in the primary circuit wastransferred to the secondary circuit which included the antenna. Theenergy was desired in the antenna circuit, whence it could be usefullyradiated; but if the primary circuit remained closed by the spark gap,energy would be transferrred uselessly back from the secondary circuitto the primary, in accordance with the well-known behavior of coupledsystems. Thus it was desirable to provide a spark gap which would openwhen the current through it first reached the low value which occurredwhen the energy had been largely transferred to the secondary circuit,and so prevent the reverse flow of energy. It was a rapid deionizationthat was wanted. The rise time was of less importance because quenchedgaps were ordinarily not used for oscillation frequencies above [.5 mHz,and a rise time of 0.l microseconds (which prior art gaps could achieve)was .a negligible fraction of the length of a half cycle. The

usual form of prior art quenched gap comprised a stack of copper disks,silver-plated in their central portions, separated by insulatingwashers, and held firmly together by a screw clamp. The disks usuallyprotruded radially appreciably beyond the washers to provide flanges fornatural or forced air cooling.

More detailed descriptions of the preceding may be found in thetechnical publications of 1910 to 1920; the first or second editions ofPrinciples of Wireless Telegraphy by .I. H. Morecroft, published by JohnWiley and Sons, discuss the behavior of coupled circuits with quenchedgaps in some detail.

The relatively recent appearance of the laser has created a need for apulse source having a very fast rise time (since laser efficiency ismarkedly a function of it) and high repetition rates in order that theaverage power may be high. For obvious reasons, simple and inexpensivestructures are desirablefor economy and, insofar as simplicity impliesabsence of powerconsuming auxiliaries, for efficiency. Semiconductor,electron tube, and mechanical devices all are lacking in some respect.

SUMMARY OF THE INVENTION:

The present invention may be described as a fixed spark gap in which aplurality of wire screen or foraminate plate electrodes. each insulatedand floating electrically, have been inserted between the main endelectrodes. The effect of this is to produce rise times comparable to aconventional two electrode spark gap, yet provide quenching as in priorquenching spark gaps so that high repetition rate pulsing can occur.Conventional spark gaps have a limited pulse rate, up to 2 kHz at most,and prior quenching spark gaps have had a slow rise time of the order of0.1 microseconds. This invention produces rise times of 2 nanoseconds,more or less depending on the peak electrical current in the pulse, yetquenches fast enough to obtain a 200 kHz repetition rate in a relaxationtype discharge circuit. This invention, therefore, has the bestcharacteristics of both the conventional spark gaps and prior quenchingspark gaps, when fast rise times and high repetition rates are desired.Pulse repetition rates far exceeding 200 kHz can be achieved in circuitsnot employing RC charging times such as in relaxation dischargecircuits.

This should not be confused with the 1.5 mHz oscillatory frequencydescribed with respect to the prior art; the maximum conventional pulserepetition rate of the prior art was in the audio range at 1,000 Hz., bythe two halves of each cycle of the output of a 500 Hz. alternator.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 represents the preferred embodiment of my invention.

FIG. 2 represents an alternate form of a part of the preferredembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

FIG. 1 represents, in central section, the preferred embodiment, whichis circularly cylindrical in form. Fixed electrodes 1 and 2 arehermetically sealed, preferably by brazing or welding, to sleeves 3 and4, respectively which are sealed to a ceramic insulating sleeve 5 whichforms the housing or envelope of the gap, and also serves to maintainelectrodes 1 and 2 fixed in position relative to each other. Electrode lis provided with a drilled path 6 to provide for the introduction ofgas, at any desired pressure, into the interior of the housing.

In the gap 7 between the opposed electrodes 1 and 2 there arerepresented two foraminate electrodes 8 and 9, having a plurality ofholes 10 and 11, respectively. For convenience, and to differentiatethem readily from electrodes 1 and 2, foraminate electrodes 8 and 9 willbe designated as baffles. Insulating washers 12, I3, and 14 serve asspacers to maintain baffles 8 and 9 in desired spacing from each otherand from the adjacent electrodes 1 and 2, respectively. In assembly ofthe device, electrode 2 may be sealed into sleeve 4. Then items 14, 9,13, 8, 12, and I may be slipped into position in ceramic insulatingsleeve 5, and, with electrode I pressed firmly against the stack ofother parts, electrode 1 may be sealed to sleeve 3. Thus all the partsindicated are fixed in position; but their fit into ceramic insulatingsleeve 5, while adequately close for mechanical stability, is not gastight, so that introduction of gas through path 6 will fill the entiredevice.

For complete representation of a use of the device, there arerepresented a high potential source connected via a resistor 16 to acapacitor 17 and one terminal of a load 18. The other terminal of load18 is connected to a terminal 19 of electrode 1. A similar terminal 20of electrode 2 is connected to ground, as is one terminal of capacitorit? and one terminal of high potential source 15. This circuit will berecognized as a conventional relaxation oscillator in which capacitor 17is charged via resistor 16 until its potential, transmitted via load 18to terminal 19, causes the gap to discharge capacitor 17 through load 18until the gap deionizes, whereupon the charging of capacitor 17 beginsagain and the cycle is repeated.

While electrodes 1 and 2 may be of tungsten (or in general of any metal)for better thermal conductivity I prefer copper, to assist in coolingthe device. Ceramic insulating sleeve may be of alumina primarilybecause such sleeves with sleeves 3 and 4 already sealed to them arecommercially available. insulating washers l2, l3, and 14 are preferablyof boron nitride both because it is readily machined and because it hasgood thermal conductivity, which facilitates cooling of baffles 8 and 9.Baffles 8 and 9 may be of copper sheet, 0.008 or 0.009 inches thick; theholes 10 and 11 in them may be 0.0l0 to 0.020 inches in diameter.However, baffles 8 and 9 may also be of wire mesh, as represented inFIG. 2, in section, as 8' and 9 respectively.

For low average operating power level, wire mesh is suitable. However,for higher average operating power levels in which cooling of thebaffles may present a problem, it is preferable to make the baffles ofmetal sheet in which a plurality of holes are present, because theeffective cross section of metal available for radial heat flow to theedge of the baffle may be greater for a given baffle thickness than ispossible with wire mesh in which the thermal contact between overlappingwires is small so that the effective cross section for heat flow isreduced. The baffles 8 and 9 should be made as thin as is consistentwith mechanical strength and thermal flow, since the field produced bypotential difference between the electrodes 1 and 2 will be reducedwithin the holes, so that they approximate drift spaces, and theacceleration of charges moving through them will be correspondinglyreduced, tending to increase the rise time of the discharge when the gapdischarges. It is also desirable to have the holes in successive bafflesin alignment (as shown in H6. 1) so that charged particles. whether ionsor electrons, may move through a succession of baffles withoutinterruption or increased path length. While this is not readilyachievable with wire mesh, plate baffles may simply be drilled in astack, and mounted in that orientation. This precaution is a refinementwhose effect will, of course, be limited by collisons, since the gap ofmy invention may be pressurized in order to increase its breakdownvoltage.

These results were obtained both with 0.004 inch diameter copper wire ina screen of 100 meshes per inch.

and with 0.00l inch diameter nickcl wire in a screen of 400 meshes perinch. The maximum pulse rate was not determined with the closest spacingof 0.005 inch because the heat dissipation caused melting of the bafflescreening before an exact figure was determined. It is evident, however,that rise time plus quenching time must have been less than 5microseconds.

It is hypothesized that the effect of the electrically 3 floatingscreens in reducing the deionizing time of a gap iwell below that of agap without the screens results afrom the following mechanism. After thedischarge lsubsides only a small potential exists across any spark j gapbut the gas is rich in ions and electrons at a highly elevatedtemperature. A charge moving toward a floating electrode will induce avoltage of like polarity in the screen or floating electrode. This willtend to repel the charge or slow it down. Conversely, a charge moving 5away from the screen will induce a voltage of opposite l polarity in thescreen, hence will attract the charge or slow it down. The voltage thatis induced in the screen may be expressed as V (q/C) (dx/dl) volts whereq is the magnitude of the charge C is the interelectrode capacitance,and

dx/dl is the path length of the charge relative to the interelectrodespacing 1 ;Note that if the screens are not floating electrically, a

current will flow and the repelling or attracting forces will not beestablished.

In any event, the charges are less energetic and the '.cross-section forrecombination becomes much larger Ethan for the high energy case, andtherefore deionization occurs rapidly. During the initiation andmaintenance of the discharge, sufficient energy is obtained 5 from theexternal power supply to overwhelm the small quenching effect.

Tests were conducted on a gap with fixed electrode geometry but avariable gas pressure. It was found that the quenching time, asevidenced by the maximum repetition rate in a relaxation dischargecircuit was independent of the gas pressure. This substantiates theabove quenching theory.

While the material of the baffles appears not to be critical fordeionization, it may be of importance to the start of the discharge.With 0.004 inch copper wire screens, the time jitter of the dischargewas marked, exceeding 15 percent of the repetition interval; and thebreakdown voltage of the gap varied little from the beginning ofoperation with continued operation even at high repetition rates. With0.001 inch nickel wire screening, the time jitter was much less; and thegap breakdown potential decreased by as much as one-half with continuousoperation. It is believed that the fine nickel wire became sufficientlyhot during the continuous operation so that the field emission producedby the application of potential between electrodes 1 and 2 was markedlyincreased, and provided rapidly a more abundant supply of electrons topromote breakdown.

Since the gap represented in FIG. 1 is made gas tight by hermeticallysealing the various interfaces between its external component parts, itis evident that it may be filled with different gases, and that it maybe pressurized.

Tests by filling with nitrogen, hydrogen, and argon gave rise times,respectively, of 1.9, 4, and nanoseconds. These compare with 2nanoseconds for air. Monatomic gases produce a large proportion ofelectronelastic collisons, with resulting low electron drift velocities,and so increase the rise time. Polyatomic gases are therefore to bepreferred. It is to be noted that while increased pressure increases thegap breakdown voltage, it does not affect the quench or deionizationtime, and thus is a convenient way of achieving the indicated result.

While the representation of FIG. 1 shows only two baffles, 8 and 9, if along total gap is required for a high breakdown voltage, the number ofbaffles should be increased to permit retention even with the longer gapof the small spacing between adjacent electrodes, that is, between theelectrodes 1 and 2 and the baffles nearest to them, and between adjacentbaffles. For a short gap (e.g. 0.020 inch exclusive of baffle thickness)one will suffice.

The fast rise time for the invention is obtained because upon initiationof the breakdown, energetic electrons and photons may traverse throughthe porous screens. Thus, rise times comparable to a conventional sparkgap are achieved as the charge multiplication (avalanche) on breakdownis present throughout the main gap. Prior quenching spark gaps withsolid disc interelectrodes, would spark over sequentially byovervoltaging each successive gap, and thus were slower in total risetime.

Cooling of the gap at high average powers is, of course. a designconsideration. My preferred way of metting this requirement is toincrease the diameter of the electrodes 1 and 2 and correspondingly ofthe bafwhich are of metal and extend to the outside so that they may becooled by various means, increases proportionally to the increase in gaparea, and their thermal resistance to proportionally reduced. It would,of course, be possible to alter the design so that the outer portions ofthe baffles extended through the housing; and the entire device could beimmersed in an insulating fluid, such as oil, to prevent externalbreakdown and convey heat. This is, however, much more complex and I donot now prefer it, although it must be recognized as a possibleexpedient for particular requirements.

Also pertinent to cooling is the fraction of the baffle area which isperforated. In screening this is of the order of one third. Ideally, theamount of solid baffle material should presumably be as little as iscompatible with the baffle producing the effects hypothesized in thepreceding; but to carry this to extremes will also reduce the materialavailable for heat flow. The fraction must therefore be a designcompromise, depending inter alia upon the magnitude of the coolingproblem.

What is claimed is:

l. A fast-rise-time quenched spark gap comprising:

a. a pair of fixed electrodes insulated from and opposed to each otherto form a gap between them. and adapted to be connected to a source ofpotential;

b. at least one foraminate electrode or baffle having its foraminateportion in the gap between the fixed electrodes, spaced uniformly apartfrom and insulated from all other electrodes,

c. in an atmosphere of gas at a pressure at least approximately 1atmosphere.

2. The device of claim 1 which comprises a plurality of foraminateelectrodes. spaced apart from and insulated from the fixed electrodesand from each other.

1. A fast-rise-time quenched spark gap comprising: a. a pair of fixedelectrodes insulated from and opposed to each other to form a gapbetween them, and adapted to be connected to a source of potential; b.at least one foraminate electrode or baffle having its foraminateportion in the gap between the fixed electrodes, spaced uniformly apartfrom and insulated from all other electrodes, c. in an atmosphere of gasat a pressure at least approximately 1 atmosphere.
 2. The device ofclaim 1 which comprises a plurality of foraminate electrodes, spacedapart from and insulated from the fixed electrodes and from each other.